Generally, a RF repeater extends the cell coverage which provides mobile communication services at low cost even in shadow areas, such as the inside of a building or underground, and that is used to increase subscriber capacity and for services in specific areas. However, in the radio frequency repeater, transmitting and receiving antennas are located at adjacent distance each other, so those transmission signals from a transmitting antenna are fed back to a receiving antenna, thereby causing the performance degradation of the repeater.
FIG. 1 is a diagram illustrating an example of a general radio frequency repeater for forward link.
As illustrated in FIG. 1, a general radio repeater includes a first antenna 10 (called donor antenna) for receiving signals to be repeated, a band-pass filtering unit 12 for filtering out unwanted frequency components from signals received from the first antenna 10, an amplification unit 14 for amplifying signals output from the filtering unit 12, and a second antenna 16 for transmitting the signals, which are amplified by the amplification unit 14, to the service area.
In mobile communication systems, such as CDMA or WCDMA, radio frequency repeaters have a forward link and reverse link. The forward link in the repeater receives radio signals from a base station, amplifies and transmits them toward a service area. On the other hand, the reverse link in the repeater receives radio signals from the service area such as mobile station, amplifies and transmits them to the base station. Since the structure and operation of the forward and reverse link are substantially the same and symmetric except the direction, only the operation of the forward link of the repeater will be described below.
In the radio frequency repeater illustrated in FIG. 1, there is a problem in that, if signals output from the second antenna 16 is strong, the transmitted signals from antenna 16 are fed back to the first antenna 10, and then amplified and are transmitted through the second antenna 16 again, thereby causing an oscillation problem. In order to avoid such oscillation phenomenon, the transmitting and receiving antennas, 10 and 16, must be placed far a part each other such that the extent of isolation between the first antenna 10 and the second antenna 16 is higher than an amplification gain, thereby there being a disadvantage in that increasing the installation cost. Furthermore, there exist constraints in which the amplification gain of a radio frequency repeater should be designed to have a decreased value in order to minimize the side effects of the aforementioned feedback phenomenon.
There have been proposed a lot of techniques to eliminate feedback signals which are electrically undesired signals in radio frequency repeaters. The basic concept of eliminating signal is to add the two signals which have the same amplitude and same frequency but have shifted 180 degree in phase. As the same procedure mentioned above, we can eliminate the feedback signals by adding the signals that have the same amplitude and frequency but have shifted 180 degree in phase to the feedback signal.
That is, because signals transmitted through the second antenna 16 are fed back into the first antenna 10, thereby generating oscillation, a method of estimating and eliminating signals transmitted through the second antenna 16 among signals received through the first antenna 10 can be implemented by estimating and subtracting signals transmitted through the second antenna 16 from signals received through the first antenna 10.
Existing technologies for eliminating undesired feedback interference signals, two techniques have been introduced. One is an analog processing technique of processing radio frequency (RF) signals without conversion into digital signals, the other technique is digital processing technique that include down converter converting first RF signal to Intermediate frequency (IF) signal, converting the IF signal to digital signal and processing the digital signal with digital signal processing techniques. After the completion of processing digital signal, the digital signals are converted to IF signal and RF signals and transmitted through transmission antenna. Generally, the analog processing method is implemented by compensating the amplitude, phase, delay, or the like of signals using dedicated hardware, so that, for the degree of flexibility is low, high cost is required and accuracy is degraded.
In contrast, in the case of digital processing method, the processing is performed using software based on digital signal processing technique, so that circuit construction is simple and the degree of flexibility is high, thereby much being used.
As an example of prior art using such technology, there are Korean Patent No. 03422002 (patented in 2002 Jul. 2, entitled “Adaptive Feedback Interference and Noise Eliminating Device), Korean Patent Publication No. 2002-0092031 (published in 2002 Dec. 11 and entitled “Radio Repeater of Mobile Communication System”), Korean patent Publication No. 2003-0069522 (published in 2002 Feb. 21 and entitled “Wideband Radio Repeater Using Interference Signal Elimination Technique of Mobile Communication System”), and Korean patent Publication No. 2003-0066062 (published in 2002 Feb. 4 and entitled (Free-oscillation Radio Repeater and Method therefore”).
Technologies for eliminating feedback signals transmitted from a transmitting antenna among signals received through a receiving antenna, which are disclosed in the above prior arts, generally include an interference signal estimation unit for estimating signals transmitted through a second antenna, an interference elimination signal generating unit for generating interference elimination signals, the amplitude and magnitude of which are identical to those of the estimated interference signals and the phase of which is shifted by 180 degrees, an interference signal eliminating unit and the like.
The above Korean Patent No. 03422002, disclosing an analog processing method which processes radio frequency signals systematically, has disadvantage in that there is limitation in adapting to noise or distortion occurring in various operational environments surrounding antennas, and the efficiency of elimination of multi-path feedback interference signal decreases because multi-path channels can not be simultaneously processed.
Furthermore, the Korean Publication No. 2002-0092031 disclosing have an initial mode prior to normal operation for estimation of feedback noise using a specific tone and followed by normal operation mode after establishing channel estimation. So aforementioned patent have disadvantage in that normal operation service should be terminated and it switches to initial mode to adapt changing environment when oscillation occurs in the normal operation mode. These phenomena degrade the service of quality even more can not service during estimation mode. So it provide multi stage inverse-phased combiner in order to reflect various operation environments, but a fixed number of multi-stage combiners implemented by hardware have limitation to accommodate various propagation environments which continuously vary.
Furthermore, the above disclosed Korean patent Publication No. 2003-0069522 adopted the technology by expanding the above-described concept to control the phase and gain in order to have the same phase for each channel respectively.
FIG. 2 illustrates another embodiment of Korean Publication No. 2002-0092031, which estimate the phase and magnitude of signals fed back to a receiving antenna using a specific tone and eliminating the feedback signals from receiving signals in receiving antenna utilizing the estimated phase and magnitude of signals, thereby preventing oscillation-related problems while the amplification gain of the radio repeater is not reduced.
Since the radio repeater illustrated in FIG. 2 the forward link and the reverse link operates in the same manner in CDMA and WCDMA systems, only the operational example of forward link can be described below. The radio repeater of FIG. 2 functions to detect the phase and magnitude of a specific tone which are transmitted through the transmission antenna and received from receiving antenna. After determining the inverse-phase by using detected specific tone information, it finally performs normal operation to eliminate the feedback signals.
The construction of FIG. 2 is described in more detail below.
First, in an initial operation state, a specific tone generated by the tone generating unit 222 of the radio repeater is inputted to a second amplification unit 224, is amplified therein, is filtered by a second filtering unit 226, and is then radiated toward a service area.
Next, the transmitting signals are received by a first antenna 200, are filtered by the first filtering unit 202, and are inputted to a phase combing unit 210. In this case, a first phase/magnitude detection unit 206 located ahead of the phase combing unit 210 detects variation values in the phase and magnitude of the specific tone and the detected variation values are input to a phase/magnitude comparator 208. The phase/magnitude comparator 208 compares the output values of the first phase/magnitude detection unit 206 with the phase and magnitude of the specific tone acquired by a second phase/magnitude detection unit 228 located subsequent to the second filtering unit 226 and transfers the comparison results to a control unit 230.
The control unit 230 transfers an inverse-phase value with respect to the specific tone, which is acquired using variation in the magnitude and phase with respect to the specific tone, to the tone generating unit 222 and an inverse-phase combing unit 212. Accordingly, the inverse-phase combing unit 212 eliminates the specific tone from receiving signals using the specific tone fed back through the first phase/magnitude detection unit 206, a gain attenuation/feedback path part 214, and a distributor 216. In this way, when the feedback signals are eliminated, a tone detection unit 218 measures the magnitude of a specific tone and, when it is less than a predetermined magnitude, informs the control unit 230 of it, and terminates the initial state operation.
Next, in the normal operational state, the radio repeater receives radio waves from a base station, amplifies them and radiates them toward a service area, and stops the tone generating unit 222, the first phase/magnitude detection unit 206 and the phase/magnitude comparison unit 208 from operating, and operates the inverse-phase combing unit 212 and the phase combing unit 210, thereby eliminating feedback signals using the phase and magnitude values acquired in the initial operation state with respect to the specific tone. At this time, the receiving signals passed through the phase combing unit 210, from which the feedback signals are eliminated, are amplified by the second amplification unit 224, are filtered by the second filtering unit 226 and are then radiated through the second antenna 232.
Furthermore, the tone detection unit 218 continues to monitor the output power of the receiving signals, from which the feedback signals are eliminated, passed through the phase combing unit 210, when the output power level of receiving signals is higher than a predetermined level, determines oscillation and transfers determination results to the control unit 230. Thereafter, the connection path between the phase combing unit 210 and the second amplification unit 224 is shut off by shutting off the switching unit 220, so that an initial state operation is performed again.
That is, the radio repeater of FIG. 2 finds out the phase and magnitude of feedback signals using a specific tone when signals radiated toward a service area are again fed back to a receiving antenna, and eliminates the feedback signals, thereby preventing the oscillation of signals.